Electromagnetic radiation detection circuit for pulse detection including an amplifying transistor and a coupling capacitor

ABSTRACT

An electromagnetic radiation detection circuit includes a photodetector transforming the received electromagnetic radiation into an electric current. A bias circuit is connected to the photodetector. An amplifying circuit has an input terminal coupled to the photodetector. An amplifying transistor has a first low-impedance electrode forming the input terminal of the amplifying circuit and a second low-impedance electrode coupled to an output terminal of the detection circuit. The transistor is configured to conduct the current applied on the first electrode. A high-impedance electric load is connected to the second electrode to deliver a voltage representative of the electric current originating from the photodetector.

BACKGROUND OF THE INVENTION

The invention relates to an electromagnetic radiation detection device.

STATE OF THE ART

Electromagnetic radiation detection devices may be used to observe aslowly-varying scene or for pulse detection, that is, to detect ahigh-power signal for a short period.

The management of a very intense light signal for a very short periodimposes many technical constraints on the detection circuit.Conventionally, the management of a high current flow for a very shorttime in the detection circuit imposes having a significant bandwidth sothat the pulse power is not time-diluted, which would make it difficultto detect. A significant bandwidth representative of a high executionspeed is generally obtained with a device having a significant electricpower consumption.

There also is a constraint relative to the sensitivity of the device tothe observed radiation in order to avoid for the pulse not to bedetected. Although the signal has a very high power, it only exists fora very short time, and it thus should be ascertained that the device iscapable of detecting the pulse.

Different embodiments have been described in prior art to detect apulsed signal. In document FR 2753796 illustrated in FIG. 1, aphotodiode 1 receives the electromagnetic radiation to be analyzed. Thecathode of photo-diode 1 is connected to a first resistor 2 and to thetwo input terminals of an amplifier 3. A second resistor 4 is connectedbetween the output terminal and one of the input terminals of amplifier3 to form a differentiating electric circuit 5. A third resistor isconnected between the cathode of the photodiode and one of the inputterminals of amplifier 3.

Such an architecture is difficult to integrate, particularly forstand-alone devices since the bias current of such a device should behigh in order to obtain a high execution speed.

In document US 2003/0205663, illustrated in FIG. 2, photodiode 1 isbiased between its substrate and a power supply voltage Vd by means oftwo field-effect transistors T1 and T2 which form a bias circuit 6. Thecathode of photodiode 1 connected to bias circuit 6 is also connected tothe gate electrode of an amplifying transistor T3. The light signalreceived by photodiode 1 is applied to the gate electrode of biastransistor T3, which amplifies this signal to send it from the drainelectrode of amplifying transistor T3 to output terminal 7. Outputterminal 7 is connected to bias circuit 6.

As in the previous embodiment, bias transistor T3 should be submitted toa significant biasing to be able to manage the amount of power deliveredby photodiode 1 with a high execution speed, which raises an electricpower consumption issue. The document is vaguer as to the followercircuit. Such a follower circuit should be slow, but no specificinformation is provided to explain how to obtain the desired slow speed.Since the bias current is defined by photodiode 1, there may be aproblem of stability of the feedback loop. Thus, such a teaching doesnot enable to viably form a pulse detector.

SUMMARY OF THE INVENTION

As can be observed, there is a need to provide a detection device whichis capable of detecting pulse signals with a decreased electric powerconsumption.

This need tends to be fulfilled by means of a device which comprises:

-   -   a photodetector transforming the received electromagnetic        radiation into an electric current,    -   a bias circuit connected to the terminals of the photodetector,    -   an amplifying circuit different from the bias circuit and        comprising an amplifying transistor formed by a field-effect        transistor or a bipolar transistor having a first source or        emitter electrode forming an input terminal of the amplifying        circuit coupled to a terminal of the photodetector and a second        drain or collector electrode coupled to an output terminal of        the detection circuit, the amplifying circuit being configured        so that the amplifying transistor be traversed by the current        applied to the first source or emitter electrode and so as to        provide this current on the output terminal with a higher        impedance than on the first source or emitter electrode,    -   a voltage source connected to the control electrode of the        amplifying transistor,    -   a passive electric load connected to the second drain or        collector electrode of the amplifying transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present inventionwill more clearly appear from the following non-limiting description ofspecific embodiments of the invention, shown in the accompanyingdrawings, among which:

FIG. 1 schematically shows a first detection circuit according to priorart,

FIG. 2 schematically shows a second detection circuit according to priorart,

FIG. 3 schematically shows a detection circuit according to theinvention,

FIG. 4 schematically shows a specific detection circuit according to theinvention,

FIG. 5 schematically shows another specific detection circuit accordingto the invention,

FIGS. 6 and 7 schematically show two still more specific embodiments ofa detection circuit according to FIG. 5.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

As illustrated in FIG. 3, the detection device comprises a photodetector1 schematized as a current source, capable of converting the receivedlight signal into an electric signal. Photodetector 1 is configured todetect an electromagnetic radiation in a specific wavelength range.

Photodetector 1 may be formed by any adapted device, for example, by aphotodiode or by a quantum well or multi-quantum well device. Thephoto-detector is advantageously configured to detect an infraredradiation, preferably, a specific range of infrared radiation, forexample, the LWIR, MWIR, or SWIR ranges.

The detection device also comprises a circuit 6 for biasingphotodetector 1. Bias circuit 6 is configured so that photodetector 1behaves as a current source for which the intensity of the currentdepends on the received electromagnetic radiation.

Bias circuit 6 imposes a first potential difference across photodetector1. The bias circuit is advantageously connected across photodetector 1.A first potential, for example, a substrate potential V_(SUB), isapplied to a first terminal of photodetector 1. A second potential, forexample, a bias potential V_(POL), is applied to the second terminal ofphotodetector 1.

Bias circuit 6 may be active or passive. Passive circuit means a circuitperforming no amplification. Such a circuit is formed by passiveelements, for example, diodes, resistors, capacitors. Active circuitmeans a circuit capable of performing the amplification, acrossphotodetector 1, of the biasing delivered by a voltage source. An activecircuit provides a better regulation of the bias point applied to thephotodetector. Such a circuit comprises at least one transistor enablingto modulate the applied voltage and it requires a higher electric powerconsumption than with a passive circuit.

Bias circuit 6 is configured to bias photodetector 1 to its desiredoperating mode. Since bias circuit 6 is coupled to photodetector 1, itis configured to avoid completely absorbing the emitted pulsed electricsignal. Bias circuit 6 and an amplifying circuit 8 are connected orcoupled to photodetector 1 to define two different flow paths for thecurrent emitted by photodetector 1.

Bias circuit 6 and amplifying circuit 8 are configured so that the pulseportion of the signal emitted by photodetector 1 mainly reachesamplifying circuit 8 and thus only slightly crosses bias circuit 6 andso that the static portion of the emitted signal mainly reaches biascircuit 6 and thus only slightly crosses amplifying circuit 8.

In such a configuration, the pulsed electric signal emitted byphotodetector 1 is representative of the received light pulse and thispulsed electric signal mainly flows through amplifying circuit 8.

To perform its biasing function, the terminal of bias circuit 6connected to photodetector 1 has a first impedance value which is low inthe low-frequency range and advantageously in the current rangeenvisaged for photodetector 1. This terminal further preferably has ahigher impedance value in the high-frequency range to prevent the lossof the current pulse signals in the bias circuit and in order to promotethe current flow inside of amplifying circuit 8.

Advantageously, the input terminal of amplifying circuit 8 has a lowimpedance in the high-frequency range and advantageously strongcurrents. It also has a higher impedance in the range of low frequenciesand advantageously of low currents

In the low-frequency range, the input impedance of amplifying circuit 8is greater than the input impedance of bias circuit 6. In the highfrequency range, the input impedance of amplifying circuit 8 is lowerthan the input impedance of bias circuit 6.

The passing from the low-frequency range to the high-frequency range isperformed around 1 MHz, typically for a value in the 0.5-10 MHz rangeaccording to the targeted type of application.

One of the terminals of photodetector 1, here, the second terminal, isalso coupled to amplifying circuit 8. Amplifying circuit 8 receives, onan input terminal, the electric signal or part of the intensity of theelectric signal emitted by photodetector 1. Amplifying circuit 8generates on its output terminal a signal representative of theinformation received on the input terminal, that is, a signalrepresentative of the information emitted by photodetector 1. The outputsignal of circuit 8 is amplified with respect to the signal present atthe input of circuit 8. The output terminal of amplifying circuit 8forms output terminal 7 of the detection device.

In a specific embodiment which may be combined with the foregoing,output terminal 7 of amplifying circuit 8 has a higher impedance thanits input terminal in order to convert the current signal originatingfrom the input terminal into a voltage signal on output terminal 7. Theinput terminal of circuit 8 is embodied by the first electrode oftransistor T3. In such a configuration, it is advantageous to maintainthe current transiting between the input terminal and the outputterminal of circuit 8 to simply obtain the desired gain.

Amplifying circuit 8 is biased by a second potential difference imposedby first and second terminals of an additional bias circuit.

The input of amplifying circuit 8 is formed by an amplifying transistorT3. Amplifying transistor T3 may be a field-effect transistor having itssource electrode or first electrode receiving the signal originatingfrom photodetector 1. The drain electrode, or second electrode, iscoupled to output terminal 7 of the detection device. The currentflowing through amplifying transistor T3 is a function and isrepresentative of the signal emitted by photodetector 1. Amplifyingtransistor T3 may also be made in bipolar technology. In this case, theemitter (first electrode) receives the signal from photodetector 1 andthe collector (second electrode) is coupled to output terminal 7.Generally, the amplifying circuit also called amplifier 8 comprises anamplifying transistor T3 having a low-impedance electrode coupled tophotodetector 1. Amplifying transistor T3 is traversed by the signal orpart of the signal emitted by photodetector 1. Advantageously,amplifying transistor T3 is connected to the photodetector to keep acompact circuit and a high execution speed. The second electrode of thetransistor is coupled to output terminal 7. Amplifying circuit 8 isconfigured so that the first electrode of transistor T3 receives,preferably, more than 50% of the current emitted by the photodetector.Amplifying circuit 8 is also configured so that the current received bytransistor T3 reaches the first electrode and is transferred to thesecond electrode and output terminal 7. Since the first electrode has alower impedance than the second electrode, there is a voltage gain.

In this embodiment, the signal emitted by photodetector 1 or arepresentative signal is applied to a low-impedance terminal, whichimproves the response time of the detection device. The source oremitter electrode of a transistor has a much lower impedance than theimpedance of the gate or base electrode of this same transistor.

Further, the signal originating from photodetector 1 generates anadditional current through amplifying transistor T3, which enables, incertain embodiments, to increase the bias current during the usefulperiod, that is, during the light pulse duration. Outside of the pulseduration, the bias current is decreased, which enables to limit theelectric power consumption of the device.

In such a configuration, the detection device may have a low biascurrent in the idle state. When the photodetector receives a lightpulse, the current pulse emitted by photodetector 1 causes the flowingof a higher current in amplifying transistor T3, which modifies the biasconditions of amplifier 8. The current emitted as a response to thelight pulse acts as an additional bias current, which enables, in thecase of an advantageously additional configuration, to increase thebandwidth of amplifier 8 at the very particular moment of the pulseoccurrence. It is then possible to decrease the bias current ofamplifier 8 during the waiting for the pulse and thus to decrease thegeneral power consumption of the circuit. Although the circuit resemblesa device mounted in direct injection mode, the operation is different.While in a direct injection circuit, the aim is to integrate electriccharges, this circuit has the function of amplifying the signal emittedby photodetector 1. To integrate these charges, the value of theelectric capacitance of the capacitor is adapted to the need and it isplaced at the circuit output. However, in this embodiment, the capacitorhas a lower electric capacitance to improve the response time.Preferably, the value of this capacitance is decreased as much aspossible to have a very low response time.

In an advantageous embodiment, because of its compactness, outputterminal 7 is formed by the drain electrode or the collector ofamplifying transistor T3.

Advantageously, amplifying transistor T3 is configured with bias circuit6 and photodetector 1 so that the current flowing through amplifyingtransistor T3 is directed in the same direction as the current takingpart in the biasing of photodetector 1. in such conditions, theimpedance value of the source electrode of transistor T3 decreases,which increases the responsiveness and thus the bandwidth of theamplifying transistor.

Due to this impedance decrease, the current injection in amplifyingtransistor T3 is improved.

The second terminal of photodetector 1 is coupled to amplifying circuit8 via a coupling circuit 9 which enables to have the signal emitted byphotodetector 1 or part of this signal pass onto the input terminal ofamplifying circuit 8.

In a specific embodiment illustrated in FIG. 4 and which may be combinedwith the previous embodiments, coupling circuit 9 is formed by aconductive line which directly connects the second terminal ofphotodetector 1 to the input terminal of amplifying circuit 8. In such aconfiguration, amplifying circuit 8 directly receives the signal emittedby photodetector 1. Only a small portion of the signal is lost in biascircuit 6 according to the performance thereof.

In another alternative embodiment illustrated in FIG. 5 and which may becombined with the previous embodiments (except that of FIG. 4), couplingcircuit 9 comprises a coupling capacitor 10 which is connected betweenphotodetector 1 and amplifying circuit 8. Coupling capacitor 10 blocksthe passing of part of the signal emitted by photodetector 1. Couplingcapacitor 10 is arranged to block the D.C. component and the slowvariations of the light signal emitted by photodetector 1 to only letthrough the fast variable component, that is, the current pulse.

Advantageously, coupling circuit 9 is formed by coupling capacitor 10,which has a first terminal connected to photodetector 1 and to biascircuit 6 and a second terminal connected to the source (or emitter)electrode of amplifying transistor T3.

In a preferred embodiment capable of being combined with the previousembodiments, the additional bias circuit comprises a first passiveelectric load 11 arranged between amplifying transistor T3 and the firstterminal of the additional bias circuit. First passive electric load 11enables to drain off the quiescent current of transistor T3. The firstpassive electric load also enables to block the pulse current to directit towards amplifying transistor T3.

In a preferred embodiment, first passive electric load 11 is configuredto avoid drifting or to limit the drift of the current representative ofthe light pulse. Such a specific architecture enables the device to bemore rapidly repositioned in a configuration where the next pulse isawaited, and thus to decrease the power consumption of the final device.In a still more specific embodiment illustrated in FIGS. 6 and 7, firstpassive electric load 11 has the electric characteristic of a diode.First passive load 11 may be formed, for example, by a diode or atransistor arranged to operate as a diode, for example, by connectingtogether the gate and drain electrodes.

First passive electric load is coupled to the source (or emitter)electrode of transistor T3.

In another embodiment which may be combined with the previousembodiments, the circuit comprises a second passive electric load 12coupled to the drain (or collector) electrode of amplifying transistorT3. The second passive electric load 12 is arranged between amplifyingtransistor T3 and the second terminal of the additional bias circuit.Second passive electric load 12 enables to offset the voltage withrespect to the associated bias line and to determine the outputimpedance value which sets the amplifier gain.

The association of passive electric load 12 with the amplifying circuitenables to convert the current signal applied to the input terminal ofthe amplifying circuit into a voltage signal on output terminal 7.

In a preferred embodiment, second passive electric load 12 is configuredto have a high impedance and a low electric capacitance. Second passiveelectric load 12 is then used to set a voltage gain of amplifyingtransistor T3 for the pulse signal.

In a specific embodiment, second passive electric load 12 behaves as acurrent source which is advantageously saturated in the absence of alight pulse. The current set point value of this current source, thatis, the intensity of the current delivered in the absence of a pulsesubstantially corresponds to the value representative of the signal tobe detected and delivered by photodetector 1. The current source isconnected to the second electrode of transistor T3.

Second passive electric load 12 is connected to output terminal 7.

The intensity of the current flowing through amplifying transistor T3 inthe idle state, that is, in the absence of a pulse, is set by thepotential applied to the control electrode of the amplifying transistor(here, the gate electrode or the base) and by the impedance value offirst passive electric load 11.

The biasing applied to the control electrode of transistor T3 originatesfrom a voltage source 14.

The embodiments illustrated in FIGS. 6 and 7 are particularlyadvantageous since they are very compact.

The terminals of photodetector 1 are connected to the bias circuit toimpose the first potential difference. Amplifying circuit 8 is formed byamplifying transistor T3. To gain more compactness, amplifyingtransistor T3 is a field-effect transistor. According to theembodiments, the source electrode is connected directly or by means ofcoupling capacitor 10 to the second terminal of photodetector 2.

First passive electric load 11 is connected to the source electrode andsecond passive electric load 12 is connected to the drain electrode.Output terminal 7 of the detection device is formed by the commonconnection between the drain electrode of amplifying transistor T3 andsecond passive electric load 12.

Advantageously, amplifying transistor T3 is of NMOS type and it iscoupled to the cathode of photodetector 1. In an alternative embodiment,amplifying transistor T3 is of PMOS type and it is coupled to the anodeof photodetector 1.

In a preferred embodiment illustrated in FIGS. 6 and 7, first passiveelectric load 11 comprises or is formed by an additional diode-connectedtransistor, for example, a diode-connected PMOS transistor. Thediode-connected transistor is connected to the common terminal of thesource electrode of amplifying transistor T3 and of photodetector 1 orcoupling capacitor 10. In addition to the above-described advantages,this architecture is particularly compact and easy to form. Thediode-connected transistor is connected between a bias line and thefirst electrode of amplifying circuit 8.

In another preferred embodiment capable of being combined with theformer, second passive electric load 12 comprises or is formed byanother additional transistor to form a current source, for example, aPMOS transistor behaving as a current source. The transistor isconnected to the drain electrode of amplifying transistor T3 to defineoutput terminal 7 of the detection device. The transistor is alsoconnected to a bias line which delivers a first voltage, for example,voltage Vdd. In the embodiment illustrated in FIG. 6, the additionaltransistor of load 12 is configured to form a diode. In the embodimentof FIG. 7, the additional transistor of load 12 has a control electrodelinked to a voltage source, which sets the maximum current delivered bythe additional transistor. In such a configuration, load 12 forms asaturated current source. The nominal current of load 12 isrepresentative of the current threshold that will be detected for apulse.

Such an architecture enables to deliver a binary signal on outputterminal 7. This signal is a function of the light pulse detected byphotodetector 1.

Second passive electric load 12 may further comprise a circuit foroffsetting the voltage levels, for example, by means of a diode, todefine the amplitude of the output signal. The diode may be formed by atransistor. The diode is assembled between current source 12 and outputterminal 7.

Bias circuit 6 of photodetector 1 advantageously comprises an additionaldiode-connected transistor connected to the second terminal ofphoto-detector 1, which provides a sufficient impedance over a wide biasrange. Advantageously, the transistor is of PMOS type, opposite to thatof the amplifying transistor (NMOS in FIG. 6) and it is connected tofirst voltage Vdd. This specific configuration is favorable for a bettertransfer of the current pulse to the amplifying transistor, which hascarriers of improved mobility. The additional diode-connected transistoris connected between a bias line and the first terminal of photodetector1.

Output terminal 7 of the detection device is intended to be connected toan analysis circuit (not shown), which memorizes the passing of thedifferent pulses. In an alternative embodiment, the analysis circuit isintegrated in the detection device. As an example, the input terminal ofthe analysis circuit is the input terminal of a flip-flop, for example,an RS flip-flop having an analog input.

As compared with the above-mentioned prior art documents, the detectiondevice enables to substantially improve the sensitivity (approximatelyby a factor 10) while allowing a very low power consumption.

The detection device is particularly advantageous when it comprises aphotodetector array. Each photodetector is then associated with a biascircuit 6 and with an amplifying circuit 8.

Electric loads 11 and 12 are output loads which collect the electricsignal provided by output terminal 7. These output loads enable to useoutput terminal 7 to output a voltage signal. Loads 11 and 12 haveimpedance values selected to deliver a voltage signal. Loads 11 and 12may independently be linear or non-linear loads. Linear load means aload having a constant value of its impedance, whatever the value of thereceived current. Non-linear load means a load having its impedancevalue varying according to the bias current. Particularlyadvantageously, the impedance value of load 11 increases when the biascurrent increases.

The invention claimed is:
 1. An electromagnetic radiation detectioncircuit comprising: a photodetector configured to transform the receivedelectromagnetic radiation into an electric current: a bias circuitconnected to terminals of the photodetector, the bias circuit receivinga first part of a current emitted by the photodetector; an amplifyingcircuit, which is different from the bias circuit, and comprising anamplifying transistor formed by a field-effect transistor or a bipolartransistor, the field-effect transistor or the bipolar transistor havinga first electrode forming an input terminal of the amplifying circuitcoupled to a first terminal of the photo-detector so as to receive asecond part of the electric current emitted by the photodetector, thefirst electrode being a source electrode or an emitter electrode, thefield-effect transistor or the bipolar transistor having a secondelectrode coupled to an output terminal of the electromagnetic radiationdetection circuit, the second electrode being a drain electrode or acollector electrode, and the amplifying circuit being configured so thatthe electric current applied to the first electrode flows through theamplifying transistor and so as to provide this electric current on theoutput terminal with an impedance higher than an impedance on the firstelectrode; a voltage source connected to a control electrode of theamplifying transistor; and a passive electric load connected to thesecond electrode of the amplifying transistor, wherein a first terminalof a coupling capacitor is directly connected to the first terminal ofthe photodetector and to the bias circuit, and a second terminal of thecoupling capacitor is directly connected to the first electrode of theamplifying transistor.
 2. The circuit according to claim 1, wherein thepassive electric load is a diode.
 3. The circuit according to claim 1,comprising a current source configured to be in a saturated state in anidle state, said current source being connected to the second electrodeof the amplifying transistor.
 4. The circuit according to claim 2,wherein the diode is assembled between a current source and the outputterminal.
 5. The circuit according to claim 2, wherein the bias circuitcomprises an additional diode-connected transistor connected between abias line and the first terminal of the photodetector.
 6. The circuitaccording to claim 1, wherein a passive load comprising an additionaldiode-connected transistor is connected between a bias line and thefirst electrode of the amplifying circuit, and is configured to form asaturated current source.
 7. The circuit according to claim 1, whereinthe voltage source connected to the control electrode of the amplifyingtransistor is configured to define an intensity of a current flowingthrough the amplifying transistor in an idle phase.
 8. The circuitaccording to claim 1, wherein the passive load comprising an additionaldiode-connected transistor is connected between a bias line and thefirst electrode of the amplifying circuit and configured to define athreshold value of a current pulse detected by the amplifying circuit.9. The circuit according to claim 1, wherein the passive load comprisingan additional diode-connected transistor is connected between a biasline and the first electrode of the amplifying circuit and configured todeliver a binary signal to the output terminal.
 10. The circuitaccording to claim 5, wherein the additional diode-connected transistoris a metal-oxide-semiconductor (MOS) transistor of a first type and theamplifying transistor is a MOS transistor of a second type opposite tothe first type so as to increase transfer of a current pulse provided bythe photodetector.
 11. The circuit according to claim 1, wherein thebias circuit and the amplifying circuit are configured so that a pulsedportion of a signal emitted by the photodetector mainly reaches theamplifying circuit and a static portion of the signal emitted by thephotodetector mainly reaches the bias circuit.
 12. The circuit accordingto claim 1, wherein the bias circuit is configured to substantially notabsorb a pulsed signal provided by the photodetector.
 13. The circuitaccording to claim 1, wherein the amplifying circuit has a low impedancein a high-frequency range and a high impedance in a low-frequency range.14. The circuit according to claim 1, wherein in a high frequency range,an input impedance of amplifying circuit is lower than an inputimpedance of the bias circuit.
 15. The circuit according to claim 1,wherein, the amplifying transistor, the bias circuit and thephotodetector are configured so that a current flowing throughamplifying transistor is directed in a same direction as a current fromthe bias circuit for the biasing of photodetector.
 16. Anelectromagnetic radiation detection circuit comprising: a photodetectorconfigured to transform the received electromagnetic radiation into anelectric current; a bias circuit connected to terminals of thephotodetector, the bias circuit receiving a first part of a currentemitted by the photodetector; an amplifying circuit, which is differentfrom the bias circuit, and comprising an amplifying transistor formed bya field-effect transistor or a bipolar transistor, the field-effecttransistor or the bipolar transistor having a first electrode forming aninput terminal of the amplifying circuit coupled to a first terminal ofthe photo-detector so as to receive a second part of the electriccurrent emitted by the photodetector, the first electrode being a sourceelectrode or an emitter electrode, the field-effect transistor or thebipolar transistor having a second electrode coupled to an outputterminal of the electromagnetic radiation detection circuit, the secondelectrode being a drain electrode or a collector electrode, and theamplifying circuit being configured so that the electric current appliedto the first electrode flows through the amplifying transistor and so asto provide this electric current on the output terminal with animpedance higher than an impedance on the first electrode; a voltagesource connected to a control electrode of the amplifying transistor;and a passive electric load connected to the second electrode of theamplifying transistor, wherein the source terminal or the emitterterminal of the amplifying circuit is connected to a terminal of thephotodetector by means of a coupling circuit configured to allow only apart of the signal emitted by the photodetector to pass through thecoupling circuit.
 17. The circuit according to claim 16, wherein thecoupling circuit has a first terminal connected to an electric nodecommon to the photodetector and to the bias circuit.
 18. The circuitaccording to claim 17, wherein the coupling circuit has a secondterminal connected to the first electrode of the amplifying transistor.19. The circuit according to claim 18, wherein the coupling circuitincludes a coupling capacitor having a first terminal connected to theelectric node common to the photodetector and to the bias circuit and asecond terminal connected to the first electrode of the amplifyingtransistor.